How Small Is the Parallax Shift of Stars?

To answer this question, you first have to understand how we measure parallax quantitatively, which is done by measuring the angle by which an object’s position appears to change with parallax; in essence, this means measuring the angle at the top of the triangle in Figure 3.3 (more technically, the “parallax angle” is defined as half of that angle). If you drew Figure 3.3 to scale, you’d find that the parallax angle for even the nearest stars is less than one arcsecond (1 arcsecond is 1/3600 of 1°; see Figure 3.9) — which is considerably less than the angular width of a human hair viewed across the length of a football field. That is why stellar parallax is undetectable to the naked eye, and can be measured only with the aid of powerful telescopes.